Mass spectrometry is an important tool in the analysis of components (or “analytes”) in a sample. In a mass spectrometric analysis, a sample has to be ionized to generate ions of the analytes; the ions are then separated based on their mass-to-charge ratios by a mass analyzer, and detected by a detector. There are many different techniques for ionizing samples, such as electrospray ionization (ESI), chemical ionization (CI), photoionization (PI), inductively coupled plasma (ICP) ionization, and matrix assisted laser desorption ionization (MALDI). Although all the techniques listed above share a common aspect, that a solid or liquid sample must be converted to a plume of molecules, atoms or ions, their mechanisms of ionization differ. As a result, the compounds that can be ionized by each of these techniques are not identical.
In the earliest implementation of electrospray, a sample plume was sprayed into a high electrical field without pneumatic or ultrasonic nebulization. This is referred to as “pure electrospray.” Pure electrospray had the problem of low flow capabilities (0.1 to 10 μl per minute). Therefore, it was difficult to use pure electrospray with liquid chromatography (LC), which has a much higher flow rate (typically up to 2 ml per minute). When the electrospray flow rate is above 100 μl per minute, it is usually impossible to maintain a sample plume, due to unstable spray formation. The ionization efficiency of pure electro spray thus decreases at higher flow rates, and sensitivity is completely lost at typical chromatographic flow rates. Therefore, the interface between LC and pure electrospray routinely splits the sample flow by a factor of 10 or more, sacrificing sensitivity, resolution and reproducibility.
The development of pneumatically assisted electrospray (or “ion spray”; see, e.g., U.S. Pat. No. 4,861,988) alleviated the flow limitation to some extent. This technique employs a concentric nebulizing gas around the central liquid delivery capillary, and enables a flow rate up to several hundred micro liters per minute, with a moderate loss of sensitivity. As discussed below, various improvements have been made to this technique.
A few years after U.S. Pat. No. 4,861,988, a heater was mounted directly on the pneumatic sprayer to assist ionization with heat and heated gas. This thermally assisted electrospray interface improved sensitivity by three times, and a flow rate of up to 500 μl per minute was demonstrated (U.S. Pat. No. 4,935,624). However, the heated nebulizer was prone to sample degradation and clogging, due to difficulty of regulating the temperature at the tip of the nebulizer.
Another implementation (Vestal, 1992) used moderately heated concentric air to assist ion formation within the electrospray plume, but, because the sprayer was deeply buried inside the concentric heated chamber, adjustment or service of the sprayer region was difficult.
At about the same time, U.S. Pat. No. 5,352,892 disclosed another way of heating the spray plume, wherein a heated disk with a central opening was placed in between a pneumatically assisted electrospray nebulizer and the ion sampling inlet to a mass analyzer. In this arrangement, a fraction of the nebulizing gas would be preheated at the opening of the heated disk body. This heated gas was then remixed with the central portion of the spray plume prior to the ion sampling inlet. In this device, heat transfer was sufficient to achieve ion formation at flow rates as high as 2 ml per minute, but the drawback was contamination of the heated disk, which required frequent cleaning.
In a design described in U.S. Pat. No. 5,412,208, the nebulization and ion sampling process was assisted by preheated gas that intersected the flow of the nebulized sample. This turbulent mixing helped to evaporate droplets of the sample, as well as push the electrospray plume in the direction of the ion sampling inlet. The main disadvantage of this design is non-uniform and limited heat exchange between the heated gas flow and the ESI plume. A newer design, described in U.S. Pat. No. 6,759,650, used two heated gas flows that intersected with the sample flow to promote turbulent mixing, but the design was complicated and less cost effective.
U.S. Pat. No. 5,495,108 discloses an ion source in which a heated drying gas is directed to a spray plume that is orthogonal to the ion sampling inlet. For example, the ion sampling inlet 236 may be positioned at 90 degrees with respect to the direction of nebulization (FIG. 2). A liquid sample 224 is delivered though a stainless steel grounded tube 226, while nebulizing gas 222 is supplied through a concentric grounded tube 228. A heated drying gas 234 is partially diverted through a special conduit 235 to deliver about 1 liter per minute of highly heated gas into the pneumatically assisted electrospray plume 237, with an overlapping ark section 243 to assist droplet evaporation and ion formation at higher sample liquid flow rates (up to 1 ml/min). The main opening 241 for the heated drying gas, defined by spray shield 238, delivers the gas at a flow rate up to 12 liters per minute. A Faraday cage electrode 239 provides a high voltage electrical field.
Another design, described in U.S. Pat. No. 7,199,364, includes a second, laminar gas flow that is heated, wherein the nozzle for the second gas flow is behind the nebulization nozzle in a semi-circular pattern. This design achieved limited heat transfer and only a moderate improvement in sensitivity.
In summary, there is a constant need for further improvements in ion source design and higher ionization efficiency.